Relays and Timers: What to Identify Before Ordering

Updated: August 6, 2026

TL;DR

Before ordering or replacing a relay or timer, collect the complete manufacturer part number, supply or coil voltage, AC or DC designation, contact arrangement, load information, timer function, timing range, mounting style, socket or terminal requirements, environmental conditions, dimensions, and required approvals. A device with the same voltage or contact count is not automatically an acceptable replacement.

What does a relay do?

A relay uses an input or control signal to change the state of one or more outputs. Relays are commonly used for control logic, signal isolation, status indication, interposing functions, switching, and other equipment-specific control tasks.

Common relay families include:

• General-purpose relays: Available in several coil voltages, contact arrangements, mounting styles, and electrical ratings.
• Plug-in or socket relays: Designed to mate with a compatible socket or base.
• Control relays: Used for control, logic, signaling, or auxiliary functions within approved systems.
• Latching relays: Designed to retain an operating state according to the device’s specified control method.
• Solid-state relays: Use electronic switching rather than movable mechanical contacts.
• Safety relays: Specialized devices intended for documented safety-related systems and functions.

These categories are not interchangeable. A general-purpose relay should not be assumed suitable for motor control, life safety, machine guarding, fire alarm, emergency control, or another specialized function.

What does a timer do?

A timer controls an output according to a defined time function. Depending on the device, that function may be triggered by the application or by the presence or loss of supply power.

Common timer descriptions include:

• On-delay: The output changes after the specified delay following the initiating input.
• Off-delay: The output remains in its operating state for the specified period after the initiating input changes.
• Interval or one-shot: The output operates for a defined period following a trigger.
• Cyclic or repeat-cycle: The output alternates according to defined on and off periods.
• Programmable timer: Provides scheduling or configurable timing functions according to the specific model.
• Astronomic timer: Uses location and date information for time-based scheduling associated with sunrise and sunset.

The name of the function alone is not enough to establish compatibility. Different timers may use different trigger methods, reset behavior, output types, timing ranges, supply requirements, programming methods, and power-loss behavior.

Start by defining the request

Tell the supplier whether you are:

• Replacing an existing device with the same part
• Replacing a discontinued device
• Repairing original equipment
• Ordering from an approved drawing or specification
• Modifying an existing control system
• Selecting a device for a new application

A verified direct replacement may be identified through the complete part number and official manufacturer documentation. New applications, substitutions, and system modifications require review by the professional responsible for the equipment and control design.

Record the complete manufacturer part number

Copy all available information from the device label, packaging, drawing, or approved documentation:

• Manufacturer
• Complete catalog or model number
• Product family or series
• Every letter, number, suffix, and option code
• Date code or revision when shown
• Previous or superseded part number
• Machine, panel, or equipment manufacturer and model

A suffix can identify a different coil, supply voltage, contact arrangement, output type, timer mode, timing range, socket, enclosure, certification, or reset function. Do not shorten the part number.

Confirm the supply or coil information

Record the exact control or supply information shown in the approved documentation:

• Rated coil or supply voltage
• AC or DC designation
• Frequency when applicable
• Permitted voltage range when provided
• Input or trigger type
• Reset or control-input requirements

Do not assume that devices marked with similar voltage numbers accept the same power type, frequency, input signal, or wiring configuration.

Identify the contacts or output

For a relay or timer with switching contacts, record:

• Number of poles
• Normally open and normally closed arrangement
• Contact form, such as SPST, SPDT, DPDT, or another documented form
• Mechanical-contact or solid-state output
• Instantaneous and timed contacts when applicable
• Contact or output ratings for the documented load type
• Required electrical isolation or common-terminal arrangement

An ampere rating by itself does not establish compatibility. The responsible designer or manufacturer must confirm that the device is suitable for the actual load, switching duty, control system, and operating conditions.

Timer information to collect

For a timer, document:

• Required timing function
• Timing range
• Required adjustment resolution
• Trigger or start method
• Reset method
• Expected behavior after power loss and restoration
• Single-function or multifunction design
• Analog dial, switches, digital display, or programmable interface
• Required scheduling, calendar, or astronomic features

Use the existing sequence of operation, approved control narrative, or manufacturer documentation. Do not determine the required timer function by bypassing controls, operating machinery, or changing settings solely for identification.

Document the mounting and connection

Record the physical configuration:

• Plug-in, socket-mounted, panel-mounted, DIN-rail, surface-mounted, flush-mounted, or other form
• Number and arrangement of pins or terminals
• Required socket, base, adapter, or retaining clip
• Terminal style
• Overall width, height, and depth
• Mounting-hole or cutout dimensions
• Available space around the device
• Display and adjustment orientation

Two devices can share the same voltage, contact form, and timer function while using different sockets, pin arrangements, dimensions, or terminal assignments.

Describe the operating environment

Provide the documented conditions around the device or enclosure:

• Indoor or outdoor location
• Ambient-temperature range
• Humidity or condensation
• Dust, debris, oil, coolant, or chemicals
• Vibration or impact
• Existing enclosure and mounting location
• Hazardous or classified-location requirements
• Any required environmental certifications

General terms such as industrial, outdoor, panel-mounted, or heavy-duty are not complete environmental specifications. Use the exact conditions and approvals shown in the equipment documentation.

Confirm approvals and safety functions

Identify any listings, certifications, manufacturer approvals, facility standards, panel requirements, or project specifications that apply to the device.

Selections involving machine guarding, emergency controls, fire alarm, smoke control, elevators, egress, process safety, hazardous locations, or other life-safety and safety-related systems require review by the responsible manufacturer, designer, qualified specialist, and Authority Having Jurisdiction as applicable.

Photos and documents that help

When the information is safely accessible, provide:

• A clear photo of the complete device label
• The front, sides, terminals, pins, or socket
• The product installed in its existing mounting location
• The panel or equipment nameplate
• Original packaging
• Approved wiring diagram or control schematic
• Sequence of operation or control narrative
• Manufacturer replacement-parts list or data sheet

Do not open energized equipment, defeat an interlock, bypass a safety device, reach into operating machinery, or energize a system solely to identify a relay or timer.

Common ordering mistakes

• Ordering from appearance alone
• Leaving suffixes off the part number
• Confusing coil or supply voltage with contact voltage
• Ignoring whether the device requires AC or DC power
• Matching only the number of contacts
• Selecting a timer without confirming its operating mode and reset behavior
• Assuming every plug-in relay uses the same socket or pin arrangement
• Using an unofficial cross-reference without manufacturer confirmation
• Treating a discontinued product’s suggested replacement as a direct substitute
• Replacing a specialized or safety-rated device with a general-purpose model

When should the selection be escalated?

Ask the equipment manufacturer, system designer, controls engineer, approved panel shop, machine-safety professional, or other responsible qualified person to review the selection when:

• The original label or part number is missing
• The device has been discontinued
• No manufacturer-approved replacement can be confirmed
• The required contact duty or load information is unclear
• The timer sequence or power-loss behavior cannot be confirmed
• The system has been modified
• The replacement requires wiring, socket, programming, or mounting changes
• The device performs a safety, emergency, interlock, fire alarm, or life-safety function
• The location is hazardous or classified
• Required approvals or system documentation are unavailable

What should you bring to Revco?

Before requesting a quote or replacement, gather as much of the following as possible:

• Manufacturer and complete part number
• Equipment, machine, or panel manufacturer and model
• Supply or coil voltage and AC or DC designation
• Contact arrangement or output type
• Documented load and duty information
• Timer function, range, trigger, and reset requirements
• Socket, pin, terminal, and mounting information
• Environmental requirements and dimensions
• Required listings and approvals
• Clear photographs and approved documentation
• Quantity and required date

Revco can use this information to search available product data or request manufacturer documentation. Final device selection, system design, settings, installation, programming, testing, and approval remain with the qualified professionals responsible for the equipment.

Browse Revco’s industrial relays, timers, and relay and timer accessories. Contact your preferred branch before visiting to discuss the product information and current availability.

About Revco Lighting & Electrical Supply

Revco Lighting & Electrical Supply operates six locations across Suffolk County, New York. Visit the Revco locations page to find the branch most convenient for your project.

Safety and design notice

This article is a product-identification and quote-preparation guide. It is not a control-system design, contact-sizing, transformer-sizing, circuit-classification, wiring, programming, installation, commissioning, maintenance, testing, or troubleshooting guide. Follow approved plans, manufacturer documentation, applicable requirements, and the direction of the qualified professionals responsible for the system. Servicing and maintenance must follow the employer’s established hazardous-energy-control program. See OSHA’s control of hazardous energy standard.